Aquaporins are a family of molecular water channels. To study protein structure, an insect cell-baculovirus expression system was developed to purify AQP4. The protein was purified to apparent homogeneity by membrane fractionation, high pH stripping, OG solubilization and anion exchange chromatography. The protein was identified by immunoblot analysis. Mass spectrometry is required to confirm AQP4 protein identity.